1. Field of the Invention
The present invention relates to a display apparatus in which a current light-emitting element is driven with a thin-film transistor (current-drive thin-film transistor display apparatus).
2. Description of the Related Art
A number of thin-film transistor display apparatuses of various types for realizing lightness in weight, smallness in size, high image qualities and high resolution have been used. Thin-film transistor display apparatuses hitherto developed, as represented by thin-film transistor liquid crystal displays, are mainly for transmission of signal voltages or transfer of minute charges. However, it is anticipated that an element capable of current driving and having a memory function will become indispensable to self-light-emitting type panels, such as EL (electroluminescence) displays, heat-developing panels and the like which are expected to be developed in future.
FIGS. 10(a) and 10(b) are an equivalent circuit diagram and a potential relationship diagram, respectively, of a current-drive thin film transistor display apparatus, in which an organic fluorescent material is used as a light emitting material.
In FIG. 10(a), symbol 121 represents a scanning line; symbol 122, a signal line, symbol 123, a common current supply line; symbol 131, a switching thin-film transistor; symbol 132, a current thin-film transistor; symbol 151, a holding capacitor; symbol 152, a pixel electrode; symbol 164, an organic fluorescent material; and symbol 165, an opposed electrode. In FIG. 10(b), line 421 represents a scanning potential; line 422, a signal potential; line 423, a common potential; line 451, a held potential; line 452, a pixel potential; and line 465, a counter potential.
The switching thin-film transistor 131 is a transistor for controlling conduction between the signal line 122 and holding capacitor 151 by a potential on the scanning line 122. That is, signal potential 422 is transmitted to held potential 451 by scanning potential 421. With respect to a displaying pixel, signal potential 422 becomes high and held potential 451 becomes high. With respect to a non-displaying pixel, signal potential 422 becomes low and held potential 451 becomes low.
On the other hand, the current thin-film transistor 132 is a transistor for controlling conduction between the common current supply line 123 and the pixel electrode 152 by the potential on the holding capacitor 151. That is, common potential 423 is transmitted to pixel potential 452 by held potential 451. With respect to a displaying pixel, conduction is effected between the common current supply line 123 and the pixel electrode 152. With respect to a non-displaying pixel, the common current supply line 123 and the pixel electrode 152 are shut off from each other.
Consequently, a current is caused to flow between the pixel electrode 152 and the opposed electrode 165 with respect to a displaying pixel, thereby causing the organic fluorescent material 164 to emit light. With respect to a non-displaying pixel, no current flows and emission of light is not caused.
Thus, the current-drive thin film transistor display apparatus has the switching thin film transistor 131 and the current thin-film transistor 132, each of which is a field effect type transistor manufactured by an ordinary semiconductor manufacturing process. As such, for the two thin-film transistors for conventional current-drive thin film transistor display apparatuses, thin-film transistors of the same structure have been used because the manufacturing cost can be reduced if the two transistors are made in accordance with the same specifications.
Actually, even if the structures of the two thin-film transistors are the same, the current-drive thin-film transistor display apparatus has no serious defect relating to it. However, if a high-quality product is aimed at, it is preferable to construct the above-described two thin-film transistors in such a manner that importance is attached to making the characteristics of the two transistors different from each other.
That is, with respect to switching thin-film transistor 131, a reduction in off current is needed for the purpose of enabling the charge to be retained more reliably in the holding capacitor 151. In contrast, with respect to current thin-film transistor 132, an increase in on current is needed for the purpose of increasing the luminance of emission of light from the organic fluorescent material 164.
However, no technical idea of positively making the characteristics of the above-described two thin-film transistors different from each other in a current-drive thin-film transistor display apparatus has been conceived.